Final Report
to the Florida Department of Natural Resources,
Bureau of Aquatic Plant Research and Control onthe Project:
Utilization of Waste water with Aquatic Mac/roplhy~ s
and the White Amur /

University of Florida
Institute of Food and Agricultural Sciences

irUIVRI TYOFLOI

ARC Fort Lauderdale Research Report Series Number FL-78-4

Final Report to the Florida Department of Natural Resources,

Bureau of Aquatic Plant Research and Control on the Project:

Utilization of Wastewater with Aquatic Macrophytes

and the White Amur

Cooperators: University of Florida, Agricultural Research Center

at Fort Lauderdale / b/

Period covering: January 1975 to December 1977

Report Prepared by:

David L. Sutton

3205 S.W. 70 Avenue

Agricultural Research Center

Fort Lauderdale, Florida 33314

a/ In cooperation with the Agricultural Research Service,

Southern Region, Florida Area, U. S. Department of Agricul-

ture and the City of Plantation Utilities Department.

b/ No portion of this report is to be reproduced in any manner

without the written consent of the University of Florida.

Table of Contents

A. Introduction . . . . .

B. Methods and Materials

1. Growth of duckweed in a sewage effluent lag

2. Harvest of the duckweed . . .

3. Sampling parameters

a. Water . . . . .

b. Plants . . . . .

C. Results and Discussion

1. Biomass production of duckweed . .

2. Water temperature . . .

3. Nutritional qualities of duckweed . .

4. Water quality . . . .

5. Estimates for grass carp production .

6. Estimates for nutrient removal . .

D. Summary and Conclusion . . .

E. Literature Cited . . . .

F. Acknowledgements . . ... ...

G. Appendix

1. Tables . . . . .

2. Figures . . . . .

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I. Utilization of Wastewater with Aquatic Macrophytes and the White
Amur

A. Introduction

A condition of accelerated eutrophication is apparent in many
bodies of water in the United States. Eutrophication of a body of
water is a process which occurs naturally during thousands of years.
The general concept of eutrophication is the nutrient enrichment of
water which results in a deterioration of fisheries and water qual-
ity. Increased production of phytoplankton, algae, and higher aqua-
tic plants limit or reduce the amount of water present in that body
of water.

Two sources of nutrient enrichment are readily evident: (1) nat-
ural and (2;) human. Some natural sources of nutrients include soil
erosion, tributary drainage from land areas, and fecal wastes from
wildlife and waterfowl. Nutrient sources as the result of human
activities constitute the most serious damage to the eutrophication
of a body of water. Urbanization and the resulting use of water-born
systems for handling municipal wastes is one of the major sources of
these nutrients.

Another major source of nutrients is wastes from livestock barn-
yards and feed lots. Pollution wastes equal to sewage from 850 mil-
lion people have been estimated for the animals that are currently
under confined feeding conditions in the United States as of 1969.
Other sources of nutrients include drainage from agricultural lands.
Some orchards have been found to lose 30 times as much nitrogen and
four times as much phosphorus as grass and woodland areas (Bartsch,
1970).

A convenient and economical method whereby these nutrients could
be removed from the water would contribute to an abatement of this
accelerated eutrophication. Additional benefits could be expected
if a method would then convert these nutrients into useable products.
The use of aquatic macrophytes for removing nutrients from enriched
water has been suggested by a number of workers (Boyd, 1968 and 1970;
Burgess, 1965, and Mackenthum, 1964).

Aquatic plants which have rapid growth rates and capabilities of
absorbing large quantities of nutrients might provide a practical and
economical method for tertiary treatment of sewage effluent. This
type of approach may be fairly inexpensive since solar energy would
be used to operate part of the process.

Waterhyacinth (Eichhornia crassipes (Mart.) Solms.) has been
suggested as a plant which could possibly remove large amounts of
nutrients. This plant is one of the most productive plants in the
world and have many desirable characteristics as a possible plant
for nutrient removal from sewage effluent. However, their sensitiv-
ity to cool weather, low nutrient and high fiber content, and diffi-
culty in harvesting are characteristics which may limit their use.
Use of waterhyacinth for livestock feeds has not been very promising.
Other aquatic macrophytes may be more desirable for a nutrient re-
moval process.

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Members of the duckweed family (Lemnaceae) are found in fresh
water habitats throughout much of the world. Duckweed grown in
water enriched with a commercial fertilizer contained approximately
35% crude protein. When these plants were fed to the grass carp,
they extracted about 80% of the crude protein from the plants (Van
Dyke and Sutton, 1977). An efficient and effective utilization of
duckweed may be possible by feeding them to the grass carp. The
grass carp could then be used for a protein concentrate or fish
meal for poultry and livestock feeds or for human consumption. A
simplified diagramatic of the use of aquatic macrophytes and grass
carp to utilize waste products generated by urban areas is presented
in Figure 1.

Nutrients from natural and human sources, and the importation of
exotic plants are two principle factors causing problems associated
with nuisance growth of aquatic plants in the United States. The
extent to which these factors interreact to produce aquatic weed
problems is not fully understood. General observations indicate that
high nutrient levels will result in an abundance of biomass from
native plants. Furthermore, in some cases exotic plants such as
hydrilla spread rather rapidly in water with rather low nutrient
levels. In the latter case, however, the water may contain an abun-
dance of only one or two mineral elements which promote growth of
these plants.

The extent to which sewage effluent from municipal wastes con-
tributes to the accelerated rate of eutrophication is difficult to
determine. Howells, et al, (1970) estimated that 16% of the summer
flow of the Hudson River was municipal sewage effluent excluding
wastes from New York City. Another report by Shapiro and Ribeiro
(1965) indicates that the sewage effluent load of the Potomac River
varies seasonally from 5 to 40% of the river volume.

The amount of sewage effluent entering a body of water would
depend on a number of factors, but in any event, with increasing
population the overall volume will continue to rise. The volume of
municipal waste is not nearly as important as the quality of efflu-
ent from the sewage treatment facility. The quality of sewage
effluent varies considerably depending primarily or the design and
capabilities of a particular treatment facility and the source of
raw sewage which it receives. Programs are under way to standardize
the quality of sewage effluent but considerable time and money will
be required to implement these changes.

The cost for cleaning sewage beyond the primary and secondary
levels is extremely high and no satisfactory method is apparently
available for tertiary treatment of sewage wastes. Exact figures
are not available, but estimates are that 450 to 680 million kg of
nitrogen and 91 to 250 million kg of phosphorus-the major mineral
elements causing increased vegetation growth in water-enter surface
waters each year from sewage effluent. These amounts are increasing
with population growth.

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In November 1977, the average daily flow of liquid effluent from
Broward County was 99.698 mgd (million gallons per day)./. In March
1975, the nutrient content of the 35 major treatment plants in
Broward County averaged 12.8 5.1 mg nitrogen per liter and 3.4 1.3
mg phosphorus per liter" Using the average daily flow of 99.698 mgd,
this represents a daily nutrient load of 10,701 pounds nitrogen (a low
of 6,427 and a high of 14,981 pounds) or a yearly amount of almost 4
million pounds of nitrogen. For phosphorus, the daily nutrient load
would be 2,837 pounds (a low of 1,735 and a high of 3,939 pounds) or
on an average yearly amount of slightly over 1 million pounds of phos-
phorus. Liquid wastes from Broward's treatment plant drain into the
extensive freshwater canal system in the County which eventually emp-
ties into the Atlantic Ocean or this effluent is pumped directly into
the Ocean. These nutrients contribute to the aquatic weed problems
in the County's waterways, as well as adding to the pollution of the
Ocean.

Utilization of sewage effluent after it has received primary and
secondary treatment, appears to be a much more feasible approach than
costly tertiary treatment. Most studies have investigated the utili-
zation of sewage effluent by terrestrial plants with very little
attention being devoted to using aquatic macrophytes for tertiary
treatment of sewage effluent. A dramatic increase in growth of a
number of terrestrial species has been observed when these plants
are watered with sewage effluent (Murphey and Brisbin, 1970 and
Sopper, 1971).- However, a greater growth response may be obtained by
growing aquatic macrophytes in sewage effluent since these plants
normally grow in a liquid media. These plants could be grown and
continually harvested in the effluent as opposed to the periodic use
of the effluent to irrigate terrestrial plants. Several workers
(Boyd, 1970; Bouwer, 1968; Pirie, 1966; and Westlake, 1963) have
suggested that aquatic macrophytes have potential for removing nutri-
ents from sewage effluent, but very few studies have been conducted
to investigate this approach.

Rogers and Davis (1972) estimated that 1.0 ha of waterhyacinth
plants under optimum growth conditions could remove the average daily
phosphorus and nitrogen wastes of over 800 people. This study was
based on a laboratory experiment where plants were grown in a stand-
ard nutrient solution and in sewage effluent grown in static and
flowing water conditions. Although waterhyacinth is a prolific
plant, other aquatic plants may be just as useful in utilizing nutri-
ents from sewage effluent. Cully and Epps (1973) suggested that the
duckweeds are another aquatic plant which may be especially useful in
utilizing nutrients in sewage effluent. Good growth of duckweed
occurred when duckweed was grown in outdoor containers filled with
domestic sewage effluent (Sutton and Ornes 1975).

An efficient and economical use of the aquatic macrophytes
is an essential feature of any design employing these plants for
nutrient removal. A number of methods have been suggested for use
of aquatic macrophytes, (Hortenstine, et al, 1971; and Little, 1968).
Most of these methods are fairly ineffective because of the large
amounts of energy which are required to remove the water, which com-
prise some 90 to 95% of the total plant weight, before the plants
may be used.

The primary purpose of this project is to determine the feasi-
bility of using floating aquatic macrophytes to remove nutrients such
as nitrogen and phosphorus from enriched water. The plants are then
fed to the grass carp to evaluate their efficiency in converting
these plants to fish proteins. Because plants take up nutrients in
different amounts and at different rates, a polyculture of plants may
be more efficient than a monoculture in removing nutrients from en-
riched water. This project is designed to study a system of aquatic
plants and the herbivorous grass carp fish for removing nutrients
from enriched waters and converting them to a useable product.

B. Methods and Materials

1. Growth of duckweed in a sewage effluent lagoon

Duckweed (Lemna minor L.), a wild strain collected from near-
by canals, was planted 23 June 1975 in the West Broward Sewage Treat-
ment Facility's effluent lagoon. This lagoon receives effluent which
has passed through an oxidation-sludge treatment process. Approxi-
mately 1 million gallons of effluent per day is processed by the
plant. The lagoon is for aeration and percolation but effluent flows
out of the lagoon at a fairly constant rate. The lagoon, itself, is
approximately 0.8 ha in size and is U-shaped, as shown in Figure 2.
The duckweed was held in place by polyvinyl chloride (PVC) pipe enclo-
sures. These enclosures were constructed with pipe 7.6 cm in diame-
ter. The first planting was with two small enclosures which averaged
35.8 0.62 m2 and two enclosures 72.4 0.2 m2. At the beginning of
the study the small enclosures were located near the outfall, as shown
in Figure 2. The larger enclosures were placed at various locations
throughout the lagoon and moved several times. However, the best
growth seemed to occur when they were located near the outfall. All
of the growth data presented in this study is from the enclosures
which were located near the outfall. Additional small enclosures
were constructed until a total of 10 had been placed in the area west
of the floating dock. The two large enclosures which were east of
the floating dock were replaced with one large enclosure constructed
with pressure-treated boards and styrofoam which enclosed an area
221 m2.

2. Harvest of the Duckweed

Duckweed was removed from the enclosures two to three times each
week with a small mesh net. Harvesting of the plants was based on the
abundance of plants within the enclosures. Estimates for dry weight
were made by removing 200-g portions of plants from that harvested from

the enclosures. These samples of plants were drained of excess
water, and then placed in a forced-air oven at 60 C, and dried to
a constant weight.

3. Sampling parameters

a. Water

Water samples were collected twice a week, Monday and
Friday, from five sampling sights in the lagoon (Figure 2). These
samples were analyzed for pH, nitrogen nitrate, and orthophosphate
phosphorus according to standard procedures.

Water temperature of the lagoon was taken at four loca-
tions along the floating dock; one on each side next to the bank
15 cm below the surface and at two sights located in the middle of
the lagoon (one 15 cm below the surface and one located at the
bottom). Air temperature was also recorded. These sites were num-
bered as follows: Site 1 the south side of the lagoon; Site 2 -
air temperature (recorder was in the shade); Site 3 the middle,
upper surface of the lagoon; Site 4 bottom of the lagoon; and
Site 5 the north side of the lagoon. These temperatures were
started between 9 December 75 and continued until 23 December.
A maximum-minimum thermometer was used to record temperature. The
temperatures were collected Monday, Wednesday, and Friday of each
week.

b. Plants

The dry duckweed was analyzed for phosphorus using the
Stannous Chloride Method (American Public Health Association,
1971). The dry duckweed samples were wet-digested with nitric and
perchloric acids prior to the phosphorus analyses.

Samples of plants collected during October 1975 to Sep-
tember 1976 were combined according to the time periods of October
to December 1975, January to March 1976, April to June 1976, and
July to September 1976. These composites of samples were then sub-
divided into four groups. The plants, labeled only with a number,
were sent to the Florida Department of Agriculture and Consumer
Service's Feed Laboratory, Division of Chemistry, Tallahassee, for
assays of moisture, protein, fat, fiber, ash, and NFE (nitrogen
free extract). The results of their analyses are presented in
Table 1, and are presented on the basis of moisture-free (100%
dry) samples.

C. Results and Discussion

1. Biomass production of duckweed

Yield for duckweed harvested from the small enclosures in
the West Broward Sewage Lagoon is presented in Figure 3. Consid-
erable fluctuation in yield is evident. The daily yield was cal-
culated to be 3.14 t 1.43 g dry weight per m for the period from

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1 September 75 to 25 September 76. The highest daily yield was
during the 7-day period of 21 to 27 June 76 when an average of
5.51 g per m2 was harvested from the 10 small enclosures.

The daily yield for the large enclosure, Number 11, was
2.21 1.46 g per m2 for the period of 29 March to 25 September
76. The highest yield was 5.80 g per m2 per day during 14 to 20
June 76.

The highest daily yield for any single enclosure occurred
during 7 to 13 June 76 when an average of 7.84 g per m2 was re-
moved from enclosure Number 8. Duckweed was not harvested from
some of the enclosures at various times because of the lack of
plants. Yield in general was highest during the spring, summer,
and fall months. High winds tended to reduce growth. On one
occasion during a rainstorm, we witnessed duckweed being blown
out of the enclosures. Duckweed was harvested weekly for a year
except during the week of 1 to 7 March 76 when no plants were
removed because of poor growth.

In the fall of 1976 the injection of chlorine for treat-
ment of the effluent was changed so that the discharge line was
near the area where the duckweed was growing. The duckweed
slowly died out of the enclosures during the remainder of the
fall of 1976 and the winter of 1977. Even though the chlorine
discharge line was soon changed back to its original position,
duckweed would not grow in the enclosures in spite of many
attempts during 1977 to replant duckweed back in the enclosures.

An estimate for production of duckweed biomass based on
various constant daily yield rates is presented in Table 2.
The daily yield rates are from 0.5 to 12 g per m2. The projec-
tion to a yield rate of 12 g per m2 may be high, but this Table
does show the possible amount of duckweed which could be produced
if the yields were constant. Maintaining the highest yield
achieved in the study of 7.8 g per m2 would result in over 24,920
kg per ha in a year or over 10 tons per acre. Through additional
studies and an improvement of cultural practices yields of duck-
weed in the upper range shown in Table 2 may possibly be achieved.
Additional studies would be required to determine these potential
growth rates of the duckweed.

2. Water temperature

Measurements of water temperature during the period of
29 December 75 to 23 December 76 are presented in Figures 5, 6, 7,
8, and 9. Air temperature at the West Broward Effluent Lagoon
ranged from a high of 36.5 C during the week 2 August 76 to a low
of 4.3 C during the week 19 January 76 (Figure 5). Water tempera-
ture closely followed the pattern for the air temperature (Figure
5).

The weekly high temperature for the south side (Site 1) of
the lagoon averaged 28.6 3.5 C (range of 20.4 to 33.9) while

that on the north side (Site 5) averaged 28.9 t 3.6 (range of
20.4 to 35.2) (Figure 6). Presented in Figure 7 are the weekly
low temperatures for Site 1 which averaged 23.4 4.2 C (range
of 13.7 to 28.9). The average difference between the high and
low was 5.2 C for Site 1. The weekly low temperature for Site
5 followed the same pattern as that for Site 1.

The upper surface (Site 3) of the center of the lagoon
averaged 29.4 3.3 and was 0.8 C higher than Site 1 (Figure 8).
However, the temperature at Site 3 closely followed that recorded
at the edges of the lagoon. The high temperature at the bottom
of the lagoon (Site 4) averaged 28.3 3.5 C (Figure 9). A dif-
ference of 1.1 C was calculated for the top and bottom of the
lagoon.

3. Nutritional qualities of duckweed

The phosphorus content of duckweed collected from enclo-
sure Number 2 and the phosphorus content of samples of effluent
collected from Site Number 4 is presented in Figure 10. Regres-
sion analysis indicated that the phosphorus content of the duck-
weed was not directly related to that found in the effluent.
Phosphorus content of the samples averaged 14.39 2.49 mg phos-
phorus per 1.0 g dry weight of plant material, while that of the
effluent averaged 3.21 0.80 vg phosphorus per 1.0 ml of efflu-
ent. In other studies (Sutton and Ornes, 1975) the phosphorus
content of duckweed was directly related to the phosphorus con-
tent in the sewage effluent up to a concentration of 2.1 pg/ml.
Since the phosphorus content of the effluent at the West Broward
Lagoon was above this 2.1 value, then a lack of correlation might
be expected.

Analyses for various feed qualities of composite samples
of weekly portions of duckweed harvested from the West Broward
Sewage Lagoon is presented in Table 1. Protein content was high-
est in the fall and lowest during the spring and summer. The
protein content in the winter was intermediate to that in the
fall and the spring-summer months. On the other hand, the fat
content of the duckweed was highest during the summer, interme-
diate during the winter and spring, and lowest in the fall.
Fiber and NFE remained constant during the year. The ash content
was highest during the summer and lowest during the fall.

The nutritional qualities compare favorably to other
well-known terrestrial plants (National Research Council Subcom-
mittee on Warmwater Fishes, 1977). For example, alfalfa meal
(Medicago sativa) may contain up to 22% protein. The duckweed
may contain up to almost 1.5 times that amount-. However, the
fiber content of duckweed was found to be about one-half that
generally present in alfalfa. Additional studies are required
to evaluate other nutritional qualities such as vitamin and amino
acid content of the duckweed.

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4. Water quality

Analyses of water samples collected from the inlet (Site
1) in the sewage lagoon contained an average of 3.62 2.51 pg
phosphorus per ml as compared to 3.20 2.28 pg per ml for those
taken from the outlet (Site 3) during the period of 21 July 75
to 22 November 76 (Figure 11). The other three sites 2, 4, and 5
contained an average of 3.28 1.94, 3.22 2.28, and 3.22 2.13
ug phosphorus per ml, respectively. These amounts are almost
identical to that contained in Site 3. There was a 0.42 pg per
ml loss in phosphorus in the effluent from the point where the
effluent entered the lagoon to the outlet. Because of the small
amount of duckweed harvested from the lagoon in relation to the
volume of effluent, it is doubtful if the removal of duckweed
contributed greatly to this loss. Perculation, uptake by algae,
phytoplankton, and other means probably contributed more to this
loss than that removed by the duckweed harvested.

Nitrogen (nitrate) was low at the beginning of the sampling
period and gradually increased (Figure 12). The concentration was
relatively uniform throughout the lagoon. Samples collected from
Site 1 contained an average of 7.09 3.55 pg per ml as compared
to 6.80 3.59 pg per ml for Site 3. Sites 2, 4, and 5 contained
an average of 6.90 3.30, 6.83 3.56, and 6.77 3.48 pg nitro-
gen (nitrate) per ml during the sampling period.

The pH of the effluent was slightly higher at the outlet
(Site 3) than the inlet (Site 1) (Figure 13). Samples collected
during the period from 21 July 75 to 22 November 77 contained an
average pH of 7.53 0.45, 7.98 0.50, 8.07 0.56, 8.10 0.55,
and 8.11 0.57 for Sites numbered 1, 2, 3, 4, and 5, respective-
ly. An increase in pH in the lagoon would be expected because of
the growth of algae and phytoplankton.

5. Estimates for grass carp production

Estimates for the amount of grass carp which could be pro-
duced from various yields of duckweed harvested from sewage efflu-
ent is presented in Table 3. The conversion efficiences are from
other studies conducted at the Fort Lauderdale Research Center
where grass carp of various sizes were fed duckweed (Sutton, 1976).
The information in Table 3 indicates the yearly amount of fish
which could be produced from 1.0 ha of duckweed if the yield of
plants and conversion efficiency remained constant. Using the
average daily yield of 3.14 g per m2 obtained in this study, the
theoretical amount of grass carp which could be produced from 1.0
ha of duckweed would be slightly more than 5,340 kg for the most
efficient conversion rate and about one-half this amount for the
least efficient rate presented in Table 3.

6. Estimates for nutrient removal

Estimates for the amount of phosphorus which could be
removed from sewage effluent as related to the harvest of duckweed

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and phosphorus content of the plants is presented in Table 4.
Using the data for the average phosphorus content of samples col-
lected from Enclosure 2 and the daily dry weight yield, phosphorus
was being removed from the effluent at an estimated rate of 45.2
mg per m2 per day or about 164 kg per ha per year. It is diff-
cult to give exact values for the amount of phosphorus which could
be removed from sewage effluent by growth of duckweed since both
yield and phosphorus content fluctuates.

The data in Tables 5 to 9 present estimates for the volume of
effluent which could be renovated of phosphorus assuming a 100%
removal of the phosphorus from a certain concentration. These
Tables were generated using the yield information and phosphorus
content of the plants. Other studies have shown that as the phos-
phorus levels in the effluent drops, both yield and phosphorus
content of the plants decreases (Sutton and Ornes, 1975). In
order to achieve a reduction in phosphorus to low levels, the con-
tact time for growth of duckweed in the effluent becomes progres-
sively longer.

It appears that two methods of expressions will be necessary
in order to express data in a meaningful manner when referring to
nutrient removal systems. These two methods have been previously
described (Sutton and Ornes, 1977). Briefly, the first method
involves expressing rates for conditions of unlimited nutrient
supply and favorable environmental conditions which result in
maximum yield and nutrient removal. The second method is to
express rates as a function of those factors necessary in order
to deplete nutrients from a supply of enriched water. The values
for these two expressions may be quite different.

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D. Summary and Conclusions

Duckweed (Lemna minor L.) was grown on the West Broward sewage
lagoon in order to evaluate growth of this plant and its ability
to remove nutrients. During the period of 1 September 1975 to 25
September 1976 an average daily yield of 3.14 g dry weight per m2
was determined for plants harvested from enclosures averaging 35.8
m2. The highest daily yield for any 7-day period was during 21 to
27 June 1976 when an average of 5.51 g per m was harvested. High
winds and rainstorms tended to reduce yield. Yield was low in the
winter months, but sufficient growth occurred for harvesting.

Air temperature at the lagoon ranged from a high of 36.5 C to
4.3 C. Water temperature closely followed the pattern for the air
temperature.

Phosphorus content of the duckweed averaged 14.4 mg phosphorus
per 1.0 g dry weight of plant material. The effluent contained an
average of 3.2 vg phosphorus per 1.0 ml of effluent. The phosphorus
content of the duckweed could not be correlated to that in the
water. Phosphorus in the effluent and in the plants fluctuated con-
siderably. Using the average values for yield and phosphorus con-
tent of the plants, phosphorus was being removed at a rate of 45.2
mg per m per day.

Analyses for nutritional qualities of the duckweed indicated
fairly high crude protein (up to 32.2%). The fiber content was
10.6%. A fat content up to 2.8% was found. There was some dif-
ferences in the nutritional quality as related to the season of
the year.

Estimates for growth of grass carp fed duckweed indicate that
plants grown under conditions such as in this might produce up to
5,340 kg of fish for each ha of duckweed. The efficiency of con-
verting these plants to fish weight by the grass carp will deter-
mine the amount of grass carp which could be produced.

This study indicates that growing duckweed on sewage effluent
may be a good method for providing a food source for rearing grass
carp. Additional studies will be required to determine the prac-
ticality of such a method for rearing grass carp for aquatic weed
control activities.

This study also indicates the need for additional research in
order to evaluate the role which other members of the duckweed
family might play in a nutrient removal system of this type. A
standardization of nutrient removal rates is needed in order to
distinguish between maximum yield and nutrient removal, and those
rates necessary to deplete nutrients in enriched water.

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E. Literature Cited

1. American Public Health Association. 1971. 13th ed. of Stan-
dard Methods for the Examination of Water and Wastewater.
N. Y. 874 pp.

This study could not have been conducted without the techni-
cal assistance of Mr. Jay Gaus, Ms. Susan Glant, Ms. Sue Heine,
Ms. Diane Johnston, Mr. Taylor Jones, Mr. Terry Lott, Ms. Jane
Michewicz, Mr. Darryl Turner, Ms. Teri Young, and Ms. Margi
VanMontfrans. Their cooperation and effort in the technical por-
tion of this study is greatly appreciated. Special thanks go to
Mr. Harold Ornes who was especially helpful with ideas in the
initial phases of the study in setting-up the sampling sites, and
constructing the floating enclosures and dock.

Thanks go to Mr. George Liner, formerly with the City of
Plantation, for his cooperation in allowing us to use the West
Broward sewage lagoon. His suggestions and comments were help-
ful in setting-up the study and interpreting data collected.

The assistance of Ms. Sharon Ramberg and Ms. Anita Reinert
in typing the reports and other correspondence related to this
study is gratefully acknowledged.

Many other individuals not listed assisted in various ways
in this study through their suggestions, comments, and handling
of the fiscal matters related to the project. Their interest and
effort in making this study possible is appreciated.

The author of this report is indebted to Dr. John Gerber for
his encouragement that this research was needed. Dr. Gerber
assisted in this study not only through his effort in providing
financial support from the Center for Environmental Programs,
but also his ideas and suggestions concerning the utilization of
aquatic plants helped to provide the stimulus necessary to carry-
out this study.

Appreciation is expressed to Dr. Alva Burkhalter for his
effort and support in providing the financial support which made
this study possible through a grant from the Florida Department
of Natural Resources, Bureau of Aquatic Plant Research and Con-
trol.

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Table 1. Analyses for various feed qualities of duckweed grown on the West Broward
Sewage Lagoon.

a/ Any two means in each column with the same letter do not differ significantly at
the 0.05% level as determined by Duncan's Multiple Range Test.

b/ Composite sample of weekly portions of duckweed harvested for each time period
and subdivided into four aliquots for analyses.

c/ NFE Nitrogen Free Extract.

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Table 2. Estimates for production of duckweed biomass based on various
daily dry weight yield rates. Harvest of duckweed is assumed
to occur three times a week from a population of plants growing
on nutrient enriched effluent from a domestic sewage treatment
plant.

Biomass produced

Daily yield
(g/m2) Kg / ha Lb / A

Week Month Year Week Month Year

0.5 35 148 1,780 31 132 1,588

1.0 70 297 3,560 62 265 3,176

2.0 140 593 7,120 125 529 6,352

3.0 210 890 10,680 187 794 9,528

4.0 280 1,187 14,240 250 1,059 12,705

5.0 350 1,483 17,800 312 1,323 15,881

6.0 420 1,780 21,360 375 1,588 19,057

7.0 490 2,077 24,920 437 1,853 22,233

8.0 560 2,373 28,480 500 2,117 25,409

9.0 630 2,670 32,040 562 2,382 28,585

10.0 700 2,967 35,600 625 2,647 31,761

11.0 770 3,263 39,160 687 2,911 34,937

12.0 840 3,560 42,720 749 3,176 38,113

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Table 3. Estimates for production of grass carp fed duckweed.
Yearly amounts of fish which could be produced are based
on various daily yield rates of duckweed and conversion
efficiencies.

Yearly fish production (kg/ha)

Daily yield
(g/m2) Conversion rate a/
2 3 4 5

0.5 890 593 445 356

1.0 1,780 1,186 890 712

2.0 3,560 2,373 1,780 1,424

3.0 5,340 3,560 2,670 2,136

4.0 7,120 4,747 3,560 2,848

5.0 8,900 5,933 4,450 3,560

6.0 10,680 7,120 5,340 4,272

7.0 12,460 8,307 6,230 4,984

8.0 14,240 9,493 7,120 5,696

9.0 16,020 10,680 8,010 6,408

10.0 17,800 11,867 8,900 7,120

11.0 19,580 13,053 9,790 7,832

12.0 21,360 14,240 10,680 8,544

a/ Amount of dry duckweed to produce a given weight of fish. For
example, a value of 2 represents a rate of fish production
whereby 2.0 g of dry duckweed will result in 1.0 g in fish
growth.

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Table 4. Estimates for phosphorus removal from sewage effluent based
on various daily dry weight yields of duckweed and phos-
phorus content of the plants.

Surface view of the West Broward Effluent Lagoon. Effluent
enters near water sampling site 1 and exits near 3. Water
sampling sites show approximate location of sample collection,
and the site number is enclosed by a circle. The small
enclosures are represented by numbers enclosed by a square.
To aid in identification of the temperature collection loca-
tions, they have been labelled as A=Site 1, B=Site 2, C=Site 3,
D=Site 4, and E=Site 5. Not drawn to scale.